Quantitative characterization of in situ letterboard use. Participants wore eye-tracking glasses that provided a video record of their field of view and their right eye’s movements. Shown here are frames from Participant 2’s processed video (produced using Yarbus software, version 2.5.0, www.positivescience.com) as he pointed to the first two letters in the word “like” in the response available in Supplementary Video 1. The red circle shows where the eye-tracker estimated he was looking; the circle was not visible during his session (see Methods). The left frame shows the end of his point to the letter “L”; the position of the red circle indicates he was also looking at “L” at that time. The middle frame shows that 374 ms later, he began fixating the letter “I” as his finger was moving toward it. The right frame shows that 510 ms after that, the participant began pointing to the letter “I” (and he continued to fixate the letter “I”). The time between the end of the point to the letter “L” and the beginning of the point to the letter “I” (the inter-point interval) was 884 ms. Ellipses represent frames not shown in the figure.